Magnetoresistive random-access memory (“MRAM”) is a non-volatile memory technology that stores data through magnetic storage elements. These elements are two ferromagnetic plates or electrodes that can hold a magnetic field and are separated by a non-magnetic material, such as a non-magnetic metal or insulator. In general, one of the plates has its magnetization pinned (i.e., a “reference layer”), meaning that this layer has a higher coercivity than the other layer and requires a larger magnetic field or spin-polarized current to change the orientation of its magnetization. The second plate is typically referred to as the free layer and its magnetization direction can be changed by a smaller magnetic field or spin-polarized current relative to the reference layer.
MRAM devices store information by changing the orientation of the magnetization of the free layer. In particular, based on whether the free layer is in a parallel or anti-parallel alignment relative to the reference layer, either a “1” or a “0” can be stored in each MRAM cell. Due to the spin-polarized electron tunneling effect, the electrical resistance of the cell changes due to the orientation of the magnetization of the two layers. The cell's resistance will be different for the parallel and anti-parallel states and thus the cell's resistance can be used to distinguish between a “1” and a “0”. One important feature of MRAM devices is that they are non-volatile memory devices, since they maintain the information even when the power is off. The two plates can be sub-micron in lateral size and the magnetization direction can still be stable with respect to thermal fluctuations.
MRAM devices are considered as the next generation structures for a wide range of memory applications. MRAM products based on spin torque transfer switching are already making its way into large data storage devices.
One of the promising MRAM technologies, OST-MRAM, uses Orthogonal Spin Transfer (OST) torque, in which orthogonal torque is applied to a free (storage) magnetic layer via a spin polarized current created by a polarizing layer (POL). Thus, the OST-MRAM device may consist of a polarizing layer, a spacer adjacent to a free layer, a free layer, an insulator layer for spin polarized tunneling, and a reference layer. The free layer, insulator and reference layer form a magnetic tunnel junction (“MTJ”). This OST configuration offers several advantages. Some of these advantages are bipolar switching, faster switching and lower write error rates for the device.
Spin transfer torque uses spin-aligned (“polarized”) electrons to change the magnetization orientation of the free layer in the magnetic tunnel junction. In general, electrons possess a spin, a quantized number of angular momentum intrinsic to the electron. An electrical current is generally unpolarized, i.e., it consists of 50% spin up and 50% spin down electrons. Passing a current though a magnetic layer polarizes electrons with the spin orientation corresponding to the magnetization direction of the magnetic layer (i.e., polarizer), thus produces a spin-polarized current. If a spin-polarized current is passed to the magnetic region of a free layer in the magnetic tunnel junction device, the electrons will transfer a portion of their spin-angular momentum to the magnetization layer to produce a torque on the magnetization of the free layer. Thus, torque can switch the magnetization of the free layer, which, in effect, writes either a “1” or a “0” based on whether the free layer is in the parallel or anti-parallel states relative to the reference layer.
FIG. 1 illustrates a magnetic tunnel junction (“MTJ”) stack 100 for a conventional MRAM device with a perpendicular polarizer. As shown, stack 100 includes one or more seed layers 110 provided at the bottom of stack 100 to initiate a desired crystalline growth in the above-deposited layers. An antiferromagnetic layer 112 is disposed over seed layers 110. Furthermore, MTJ 130 is deposited on top of synthetic antiferromagnetic (SAF) layer 120. MTJ 130 includes reference layer 132, which is a magnetic layer, a non-magnetic tunneling barrier layer (i.e., the insulator) 134, and the free layer 136, which is also a magnetic layer. It should be understood that reference layer 132 is actually part of SAF layer 120, but forms one of the ferromagnetic plates of MTJ 130 when the non-magnetic tunneling barrier layer 134 and free layer 136 are formed on reference layer 132. As shown in FIG. 1, magnetic reference layer 132 has a magnetization direction parallel to its plane. As also seen in FIG. 1, free layer 136 also has a magnetization direction parallel to its plane, but its direction can vary by 180 degrees.
The first magnetic layer 114 is disposed over seed layer 110. SAF layer 120 also has an antiferromagnetic coupling layer 116 disposed over the first magnetic layer 114. Furthermore, a nonmagnetic spacer 140 is disposed on top of MTJ 130 and a polarizer 150 is disposed on top of the nonmagnetic spacer 140. Polarizer 150 is a magnetic layer that has a magnetic direction perpendicular to its plane and orthogonal to the magnetic direction of the reference layer 132 and free layer 136. Polarizer 150 is provided to polarize a current of electrons (“spin-aligned electrons”) applied to MTJ structure 130. Further, one or more capping layers 160 can be provided on top of perpendicular polarizer 150 to protect the layers below on MTJ stack 100. Finally, a hard mask 170 is deposited over capping layers 160 and is provided to pattern the underlying layers of the MTJ structure 100, using a reactive ion etch (RIE) process.
Conventional MRAM devices such as those described in U.S. Pat. No. 6,532,164 to Redon describe embodiments having a nonmagnetic metallic conductor layer in between the polarizer layer and MTJ. One of the key problems of the approach in both Redon and all other such OST MRAM devices is the lack of a control of orthogonal torque transfer and efficiency of spin transfer torque through metallic (i.e., conductive) spacers 140 separating free layer 136 of the MTJ from the polarizing layer 150.
In devices such as Redon, the spacer layer 140 was constructed with a high resistivity non-magnetic metal such a Ta or a low resistivity transition metal such as Cu, both of which have disadvantages. High resistivity non-magnetic metals such as Ta are known to have short spin diffusion length, e.g., approximately one nanometer, which suppresses spin transfer torque from the polarizing layer 150. On the other hand low resistivity transition metals such as Cu provide very good spin torque transfer due to a long spin diffusion length (250 nm at room temperature). However the high tunnel magnetoresistance (TMR) ratio decreases significantly when Cu is used as a spacer between the polarizer and the MTJ due to Cu thermal diffusion. This leads to poor performance of the MTJ device.